May 12, 2013

The female′s external reproductive structures are the
clitoris and two sets of labia, which surround the clitoris and vaginal
opening. The internal organs are a pair of gonads and a system of ducts and
chambers that carry gametes and house the embryo and foetus.

OvariesThe female gonads, the ovaries, lie in the abdominal cavity,
flanking, and attached by a mesentery to, the uterus. Each ovary is enclosed in
a tough protective capsule and contains many follicles. A follicle consists of one egg cell surrounded
by one or more layers of follicle cells, which nourish and protect the
developing egg cell. Most or all of the 400,000 follicles a woman will ever
have are thought to be formed before her birth. Only several hundred follicles will release egg cells during a
woman′s reproductive years. Starting at puberty and continuing until menopause,
usually one follicle matures and releases its egg cell during each menstrual
cycle. The cells of the follicle also produce the primary female sex hormones,
the estrogens. The egg cell is expelled from the follicle in the process of ovulation. The
remaining follicular tissue then grows within the ovary to form a solid mass
called the corpus luteum
(“yellow body”). The corpus luteum secretes additional estrogens and
progesterone, a hormone that helps maintain the uterine lining during
pregnancy. If the egg cell is not fertilised, the corpus luteum disintegrates,
and a new follicle matures during the next cycle.

Oviducts and UterusThe female reproductive system is not completely closed, and
the egg cell is released into the abdominal cavity near the opening of the oviduct, or
fallopian tube. The oviduct has a funnel–like opening, and cilia on the
epithelium lining the duct help collect the egg cell by drawing fluid from the
body cavity into the duct. The cilia also convey the egg cell down the duct to
the uterus,
also known as the womb. The uterus is a thick, muscular organ that can expand
during pregnancy to accommodate a 4–kg foetus. The inner lining of the uterus,
the endometrium,
is richly supplied with blood vessels. The neck of the uterus is the cervix, which
opens into the vagina.

Vagina and VulvaThe vagina
is a thin–walled chamber that is the repository for sperm during copulation and
that serves as the birth canal through which a baby is born. It opens to the
outside at the vulva,
the collective term for the external female genitalia.

At birth, and usually until sexual intercourse or vigorous
physical activity ruptures it, a thin piece of tissue called the hymen partly
covers the vaginal opening in humans. The vaginal opening and the separate
urethral opening are located within a recess called the vestibule, bordered by a pair of slender skin
folds, the labia minora.
A pair of thick, fatty ridges, the labia majora, encloses and protects the labia
minora and vestibule. Located at the front edge of the vestibule, the clitoris
consists of a short shaft supporting a rounded glans, or head, covered by a
small hood of skin, the prepuce. During sexual arousal, the clitoris, vagina,
and labia minora all engorge with blood and enlarge. The clitoris consists
largely of erectile tissue. Richly supplied with nerve endings, it is one of
the most sensitive points of sexual stimulation. During sexual arousal, Bartholin′s glands,
located near the vaginal opening, secrete mucus into the vestibule, keeping it
lubricated and facilitating intercourse.

Mammary GlandsMammary
glands are present in both sexes but normally function only in
women. They are not part of the reproductive system but are important to
mammalian reproduction. Within the glands, small sacs of epithelial tissue
secrete milk, which drains into a series of ducts opening at the nipple. Fatty
(adipose) tissue forms the main mass of the mammary gland of a non-lactating
mammal. The low level of estrogen in males prevents the development of both the
secretory apparatus and the fat deposits, so male breasts remain small, and the
nipples are not connected to the ducts.

Male
Reproductive AnatomyIn most mammalian species, including
humans, the male′s external reproductive organs are the scrotum and penis. The
internal reproductive organs consist of gonads that produce gametes (sperm
cells) and hormones, accessory glands that secrete products essential to sperm
movement, and ducts that carry the sperm and glandular secretions.

TestesThe male gonads, or testes
(singular, testis),
consist of many highly coiled tubes surrounded by several layers of connective
tissue. These tubes are the seminiferous tubules,
where sperm form. The Leydig cells that are
scattered between the seminiferous tubules produce testosterone and other
androgens.

Production of normal sperm cannot occur at
the body temperatures of most mammals, and the testes of humans and many other
mammals are held outside the abdominal cavity in the scrotum,
which is a fold of the body wall. The temperature in a scrotum is about 2°C
below that in the abdominal cavity. The testes develop high in the abdominal
cavity and descend into the scrotum just before birth. In many rodents, the
testes are drawn back into the abdominal cavity between breeding seasons,
interrupting sperm maturation. Some mammals whose body temperature is low
enough to allow sperm maturation, such as monotremes, whales, and elephants,
retain the testes within the abdominal cavity permanently.

DuctsFrom the seminiferous tubules of a testis,
the sperm pass into the coiled tubules of the epididymis.
It takes about 20 days for sperm to pass through the 6–m–long tubules of each
epididymis of a human male. During this passage, the sperm become motile and
gain the ability to fertilise an egg. During ejaculation,
the sperm are propelled from the epididymis through the muscular vas deferens. These two ducts (one from each
epididymis) run from the scrotum around and behind the urinary bladder, where
each joins a duct from the seminal vesicle, forming a short ejaculatory duct. The ejaculatory ducts open
into the urethra, the tube that drains both
the excretory system and the reproductive system. The urethra runs through the
penis and opens to the outside at the tip of the penis.

GlandsThree sets of accessory glands—the seminal
vesicles, prostate gland, and bulbourethral glands—add secretions to the semen, the fluid that is ejaculated. A pair of seminal vesicles contributes about 60% of the
total volume of semen. The fluid from the seminal vesicles is thick, yellowish,
and alkaline. It contains mucus, the sugar fructose (which provides most of the
energy used by the sperm), a coagulating enzyme, ascorbic acid, and
prostaglandins, local regulators discussed in Chapter 45.

The prostate
gland is the largest of the semen–secreting glands. It secretes its
products directly into the urethra through several small ducts. Prostatic fluid
is thin and milky; it contains anticoagulant enzymes and citrate (a sperm
nutrient). The prostate gland is the source of some of the most common medical
problems of men over age 40. Benign (noncancerous) enlargement of the prostate occurs
in more than half of all men in this age–group and in virtually all men over
70. Prostate cancer is one of the most common cancers in men. It is treated
surgically or with drugs that inhibit gonadotropins, resulting in reduced
prostate activity and size.

The bulbourethral
glands are a pair of small glands along the urethra below the prostate.
Before ejaculation, they secrete a clear mucus that neutralizes any acidic
urine remaining in the urethra. Bulbourethral fluid also carries some sperm
released before ejaculation, which is one reason for the high failure rate of
the withdrawal method of birth control.

Semen in the Female Reproductive TractA man usually ejaculates 2–5 mL of semen,
and each milliliter may contain 50–130 million sperm. Once in the female
reproductive tract, prostaglandins in the semen cause thinning of the mucus at
the opening of the uterus and stimulate contractions of the uterine muscles,
which help move the semen up the uterus. The alkalinity of the semen helps
neutralize the acidic environment of the vagina, protecting the sperm and
increasing their motility. When first ejaculated, the semen coagulates, making
it easier for uterine contractions to move it along; then anticoagulants
liquefy the semen, and the sperm begin swimming through the female tract.

PenisThe human penis
is composed of three cylinders of spongy erectile tissue derived from modified
veins and capillaries. During sexual arousal, the erectile tissue fills with
blood from the arteries. As this tissue fills, the increasing pressure seals
off the veins that drain the penis, causing it to engorge with blood. The
resulting erection is essential to insertion of the penis into the vagina.
Rodents, raccoons, walruses, whales, and several other mammals also possess a baculum, a bone that is contained in, and helps
stiffen, the penis. Temporary impotence, a reversible inability to achieve an
erection, can result from alcohol consumption, certain drugs, and emotional
problems. Several drugs and penile implant devices are available for men with
nonreversible impotence due to nervous system or circulatory problems. The oral
drug Viagra promotes the action of the local regulator nitric oxide (NO),
enhancing relaxation of smooth muscles in the blood vessels of the penis. This
allows blood to enter the erectile tissue and sustain an erection.

The main shaft of the penis is covered by
relatively thick skin. The head, or glans penis,
has a much thinner covering and is consequently more sensitive to stimulation.
The human glans is covered by a fold of skin called the foreskin, or prepuce, which may be removed by circumcision.